The present invention relates to integrated transmission and switching systems and more particularly to a communications processor.
It is desirable in telephone switching systems to incorporate sophisticated integrated switching techniques to enable switching of telephone calls at a "local" level thereby enabling connections of a local subscriber line to the outside world.
Referring to the cross-referenced application entitled "Integrated Transmission and Switching System", a pulse code modulated (PCM) switching system is disclosed, having a base switch and one or more subscriber switches to enable connections between a plurality of local subscriber lines and the outside world, utilizing a plurality of multitime slot digital data buses commonly known as a T-1 line. In PCM switching techniques analog data is sampled at 8 Khz and the resultant sampled data are multiplexed within a periodic time frame. One PCM switching technique utilizes a multi-frame format of 12 frames where each frame includes 24 time slots at a periodic frame rate of 125 .mu.sec.
Each time slot utilizes 8 bits of sampled information to digitally represent the analog information. In a multi-frame format of 12 frames, each frame includes 193 bits per frame and frames 6 and 12 are the signaling frames where the sixth frame is called the A frame and the twelfth frame is called the B frame. During the sixth and twelfth frames of the multi-frame format, the eighth bit (the least significant bit) is utilized for supervisory signaling purposes such as indicating on-hook, off-hook and dial pulsing conditions. Generally, there is no signal degradation if only one of the frames is to be utilized for signaling purposes. It is, therefore, desirable to utilize the signaling bit in the B frame (frame 12) for other purposes, such as a common data channel. This would provide a data channel comprising 24 bits.
Alternatively, another data channel could be utilized by using the framing bit which occurs at the beginning of each frame in a multi-frame format, which identifies the beginning of a particular frame. Odd frames in the multi-frame format (F1, F3, F5, F7, F9, F11) are indicated by framing bit code of alternating "1" and "0". The framing code for odd frames is, therefore, 101010. To identify the signaling frames in a multi-frame format (F6 and F12), the even frames in a multi-frame format utilize a different code to identify the signaling frame. Whenever there is a transition from one frame to a signaling frame in the even frame format, the framing bit changes state. Therefore, the even frame code for frames F2, F4, F6, F8, F10, F12, is 001110. The transition between frame 4 and frame 6 is a change of state to indicate a signaling frame. Similarly, the transition between frame F10 and F12 is a change of state from "1" to "0". The entire framing code is 100011011100. The even frame pattern can be seen as a 000111 code which is required to check the sixth and twelfth frame for desired signaling purposes. It is desirable to utilize the framing code to establish a data channel by periodically removing the 000111 even framing pattern and substituting, therefore, a data channel.
The cross referenced application entitled "Integrated Message Accounting System" utilizes a signal processor to detect dial pulsing, on-hook and off-hook conditions and to send supervisory signaling bits to specified time slots during the signaling frames. In order to enable the connections between a subscriber line and the outside world, there is a need for a communications system between a central office base switch and a local subscriber switch to provide the necessary communications link. This results in the desirable feature of utilizing existing equipment while providing a communications link between a subscriber switch and the central office base switch.
In view of the above background, it is an object of the present invention to provide a communications processor for establishing a communications link between a central office base switch and a local subscriber switch.